1. Field of the Invention
This invention relates to a nonwoven-sheet-making process in which each of a plurality of fiber streams is oscillated as it is forwarded to a moving receiver on which it deposits its fibers to form a ribbon which combines with ribbons formed by other streams. In particular, the invention concerns an improved process in which the oscillation frequency of the fiber stream is varied to provide an improvement in the uniformity of the resultant sheet.
2. Description of the Prior Art
Many processes are known wherein fibers from a plurality of positions are deposited and intermingled on the surface of a moving receiver to form a wide nonwoven sheet. For example, Knee, U.S. Pat. No. 3,402,227, discloses a plurality of jets positioned above a receiver and spaced in a line that makes an angle with the direction of receiver movement so that the fiber streams that issue from the jets deposit fibers on discrete areas of the receiver to form ribbons which combine with ribbons formed from other streams along the line. Also, several methods are known for directing the fibers from a plurality of positions to various locations across the width of the receiver. Frickert, U.S. Pat. No. 2,736,676, for example, discloses directing glass fibers to a receiver by means of a wobble plate or by means of a cylinder which rotates about an axis that is canted at a small angle to the longitudinal axis of the cylinder. Steuber, U.S. Pat. No. 3,169,899, discloses the use of curved oscillating baffles for spreading flash-spun plexifilamentary strands while oscillating and directing them to a moving receiver. Processes for flash-spinning the plexifilamentary strand are disclosed in Blades and White, U.S. Pat. No. 3,081,519.
An efficient method for depositing fibers onto the surface of a moving receiver is disclosed in Pollock and Smith, U.S. Pat. No. 3,497,918. In a preferred embodiment of Pollock and Smith, plexifilamentary strand is flash-spun and forwarded in a generally horizontal direction into contact with the surface of a rotating lobed baffle. The baffle deflects the strand and accompanying expanded solvent gas downward into a generally vertical plane. Simultaneously, the baffle spreads the strand into a wide, thin web and causes the web to oscillate as it descends toward the receiver surface. An electrostatic charge is imparted to the web during its descent to the receiver. The web is then deposited as a wide swath on the surface of the receiver. To make wide sheet, numerous flash-spinning units of this type are employed. The units are positioned above the moving receiver surface so that the deposited swaths form ribbons which partially overlap and combine to form a multi-layered sheet.
Multi-position apparatus of the type disclosed in Pollock and Smith has been very useful in commercial production of wide nonwoven sheets prepared from flash-spun plexifilamentary strands. In the past, such apparatus has been operated with all of the baffles rotating at substantially the same constant speed. However, such operation was sometimes accompanied by the formation of lanes of high and low unit weight or thickness in the sheet. These lanes through hardly measurable on one layer of sheet, became visible as "gauge bands" in rolls of the sheet, wherein many layers of sheet are wound up, one atop the other. The gauge bands, in turn, apparently caused uneven or telescoped edges of the roll. Because of compressive forces exerted by the wound-up sheet, the lanes of higher unit weight or thickness became denser than other parts of the sheet in the roll. Subsequently, such differences in density often led to nonuniformities in printing or dyeing of the sheet.
The purpose of the present invention is to eliminate or at least significantly reduce the formation of deleterious gauge bands in nonwoven fiber sheet.